This invention relates to apparatus for generating an abrasive fluid jet and especially to such apparatus for use in cleaning and cutting operations.
Prior apparatus of this sort includes a jetting head for ejecting a jet of abrasive particles entrained in a stream of a fluid which may be either gas or liquid, normally either air or water. The fluid is normally passed axially through the head as a high pressure jet and the abrasive particles are introduced into the fluid jet generally at an angle to the direction of flow thereof and are entrained in the jet so that a jet of abrasive particles is ejected at high speed from the outlet of the head onto the article to be abrasively treated. The abrasive particles are introduced into the head in flowable form and may be either dry, in which case they are usually fed into the jet already entrained in air, or wet, in which case they are normally fed into the jet in the form of a liquid slurry. In most hitherto known apparatus the head operates as a jet pump in that the venturi effect of the fluid jet passing through the head draws the abrasive particles into the head.
The hitherto known apparatus has a number of disadvantages which primarily arise because the head operates as a jet pump. First, the concentration of abrasive particles that can be entrained in the fluid jet is limited by the pumping capacity that can be generated thereby and moreover it is extremely difficult to ensure a constant concentration of abrasive particles in the jet. Secondly, the source of abrasive particles must be close to the head because the jet pump cannot draw the abrasive particles through long runs of piping. Thirdly, when the fluid jet is reduced or switched off the pump action drops to such levels that the abrasive particles settle in the feed pipe thus causing blockage of the feed pipe. Such blockages are virtually impossible to clear simply by re-generating the fluid jet to recreate the jet pump effect.
Because of the low concentration of abrasive material and the inconsistencies in the concentration it has not proved possible with the hitherto known apparatus to provide an abrasive jet with the capacity for cutting hard materials such as stone and metals and the use of the hitherto known apparatus has generally been limited to cleaning operations and the cutting of soft materials.